This is based on The Little Princess, which I've neither seen nor read, but someone described how great an AU it would be, so I looked up the last scene on youtube and here we are

Anna hears voices shouting from outside. She darts of the room - maybe they won't find her if she's not hiding by the window where she fell in? She barrels through the doorway and is huddling in the corner when she notices the person sitting by the fire, now staring at her with wide eyes.

Her heart stops - okay it doesn't actually stop but her breath catches and she can't move and she feels funny all over. Anna just stares at her. Because she's supposed to be dead. Supposed to be been dead for years.

"Elsa," Anna finally manages to gasp. She's getting over the initial shock and clambers forward, towards the chair where her big sister is sitting. "Elsa!"

The older girl furrows her brown in confusion. "I'm sorry… do I know you?"

The words echo like they're coming down a long tunnel, and she thinks maybe she hasn't heard them properly. Her heart clenches painfully, and she manages to choke out. "Elsa, it's me. Anna."

The girl in the chair shakes her head of braided blonde hair. She's older than Anna remembers, but of course she is, it's been years. Her face is lined and worried. But it's Elsa.Anna would never mistake her, never forget…

Forget…?

Had Elsa forgotten her?

"Do you remember me?" Anna asks, and again, the girl shakes her head.

"Should I?"

Somewhere beneath Anna's shock, things start to make sense - Elsa would never forget her on purpose, but if something happened to her… that would explain, all these years, why she had never come for Anna, why they'd all thought her dead. It wasn't a lie, not exactly. Anna tries again, seeing the blank stare on her sister's face.

"I'm your sister. Your little sister. Anna. You… you left to go with father and mother to India, but none of you ever came back. They told me you'd died."

"India…" she murmurs. "Yes, they told me I'd been there before. But I can't remember that, either." She's still staring at Anna's face, like there's some hidden detail that will spark her memory. The shouting outside grows, then is joined by heavy pounding at the door. Anna rushes forward and takes the older girl's hands.

"Elsa, please, remember. They're going to take me away, and I'll never see you! again"

She's taken aback at the sudden contact, but doesn't pull away from Anna. The door bursts open, and three policemen and a surly-looking older woman rush in. Two of the police officers grab Anna and start to drag her away, but she puts up a fierce struggle.

"No!" she shrieks. "Elsa! Elsa, please! Tell them who you are! You're my sister!"

Anna looks to the woman, but she glares impassively. "Take the child away," she directs the officers, and they drag Anna kicking and screaming back out into the rain.

Elsa stares at the bizarre scene. Her head is pounding. The young girl's face is like a whisper on her thoughts that she can't properly hear. She says they're sisters, but she can't… can't remember—

"Elsa!" The front door is open and she can faintly hear the girl screaming from outside. "Don't leave me!"

And suddenly something clicks—Elsa is still staring at the fireplace, but she sees a house on the water, and two parents, and a small girl tugging on her skirts and frowning and begging her not to leave—

The officers have nearly forced Anna into the police car when the older girl comes rushing out of the front door. Her thoughts are a jumble of faces and scenes she can't sort out yet, but this one face she knows again as well as her own.

"Anna!"

The officers jump at the sudden cry, and Anna wriggles out of their grasp and flies up the steps to the girl waiting with open arms.

"Anna!" Elsa cries again as her younger sister jumps into her arms and clutches her, their faces buried in each others's shoulders. The rain is heavy, but they don't care, it could be hail or snow or ice and they would still stand there holding each other, crying out years of loneliness. "Anna," Elsa whispers again so she'll never forget it. "Anna. I'm so sorry. I won't leave you ever again."

"You found me," Anna holds on tighter. "They told me you were dead and you were never coming back, but I knew you'd find me."

They stand there, crying and laughing in the rainstorm until one of the officers finally drops a coat over them and ushers them back into the house. Anna refuses to let go, and that is fine with Elsa, who carries her back in, still whispering her name so she won't ever forget it again.